The Milky Way Our home galaxy, full of stars, gas and mysterious dark matter We decompose it into a disk and a halo and a few other parts

Almost a View of our Milky Way NGC 4526, a spiral galaxy like the MW but about 30 Mpc away: it has a similar size, luminosity and structure

Edge-on “View” and View of MW

Key Parts of the Milky Way HALO Contains most globular clusters, and most Pop II stars; roughly 30 kpc (105 lt-yr) in diameter. THICK DISK roughly 5 kpc thick, and 30 kpc in diameter; contains Pop I stars (but low density). THIN DISK ~500 pc thick: contains MOST stars; includes spiral arms and great majority of luminosity. DUST DISK only 50 pc thick; new stars are born in the molecular clouds found within this very thin disk. SPIRAL ARMS are wrapped within the dust/thin disk; contain almost all hot, luminous (O and B) stars.

Overall Structure of the Milky Way

Inner Parts GALACTIC BULGE roughly 2 kpc in radius around center; highest concentration of stars, including many globular clusters. GALACTIC CENTER in the direction of the constellation Sagittarius, some 8 kpc from the Solar System (SS). Multiwavelength Milky Way Start here on 11/13

MAPPING the MILKY WAY Dust, mainly in molecular clouds, shrouds the Disk; we see few stars beyond 2 kpc from SS in the thin disk, where the number of stars is much greater Originally astronomers thought the Milky Way WAS the Whole Universe & SS central to it (because of visible light extinction by dust) Location of Globular Clusters in halo implied center towards Sagittarius and SS actually towards one side in early 20th century. Atomic Hydrogen gas sends 21 cm radio waves that allow us to map the far side of the galaxy, and the outer reaches where there are few stars

Our Nearest Big Neighbor, M31, the Andromeda Galaxy Andromeda, about 30 kpc across down to nucleus only 15pc

A Limited Conception of the MW Herschel’s “map” of the Galaxy from star counts; More in the MW plane, but thought the Sun near the center and got the size too small: didn’t understand dust

Distribution of Globular Clusters

How do stars orbit in our galaxy?

Stars in the disk all orbit in the same direction with a little up-and-down motion

Orbits of stars in the bulge and halo have random orientations

Thought Question Why do orbits of bulge stars bob up and down? A. They’re stuck to interstellar medium B. Gravity of disk stars pulls toward disk C. Halo stars knock them back into disk

Thought Question Why do orbits of bulge stars bob up and down? A. They’re stuck to interstellar medium B. Gravity of disk stars pulls toward disk C. Halo stars knock them back into disk